Severe Demyelinating Polyneuropathy and Cranial Neuropathy During Avelumab Treatment of Metastatic Merkel Cell Carcinoma

Introduction Avelumab is a programmed death ligand 1–blocking monoclonal antibody used for the treatment of Merkel cell carcinoma (MCC), urothelial carcinoma, and other solid tumors. It acts as an immune checkpoint inhibitor and prolongs survival of MCC patients. Immune-mediated neurological adverse effects are rare and usually respond well to specific therapy. Methods and Results A case of a 70-year-old man with metastatic MCC is described in this study. The patient developed diplopia after the fourth dose of avelumab, which was then discontinued. Seven months later, therapy was reinitiated and followed by a new adverse neurological event: severe demyelinating polyneuropathy combined with ophthalmoplegia refractory to a plethora of immune suppressive/modulatory treatment regimes. Discussion This report of severe demyelinating polyneuropathy and cranial neuropathy associated with an anti–programmed death ligand 1 drug refractory to immune suppressive/modulatory treatments sheds a new light to evolving spectrum of immune checkpoint inhibitor immune-related neurological adverse events.